When light levels are low, the pixels of a matrix image sensor gather only a small number of electrons and it is necessary to greatly increase the integration period to obtain an image—but this reduces the signal-to-noise ratio.
In charge coupled device (CCD) technology it has already been proposed to incorporate electron multiplication systems into sensors, systems which create additional electrons from the electrons generated naturally by the light. The electrical signal that is then gathered is therefore multiplied by a coefficient. The noise also increases but to a lesser degree than the signal.
This principle of electron multiplication in CCD technology consists in increasing the potential differences present between the charge-transfer gates, thereby accelerating the electrons during transfer. The energy which is given to these electrons is sufficient for impacts with atoms of the semiconductor material to make electrons of these atoms pass from the valence band to the conduction band. These electrons torn from the atoms are themselves accelerated and may cause other impacts. This results in electron multiplication.
In CCD sensors it is possible to do this because the electrons are transferred from gate to gate and it is the increase in the voltage of certain gates which allows the electrons to be greatly accelerated so as to cause this multiplication.
But in active-pixel sensors, comprising within each pixel a charge-voltage conversion circuit (a few transistors), this is not possible because the electron packets are converted into a voltage immediately after each integration period. They are not transferred from gate to gate.